1. Field of the Invention
The present invention relates to a laser beam irradiation method and a laser irradiation apparatus for using the method (apparatus including a laser and an optical system for guiding laser beam emitted from the laser to an object to be illuminated). In addition, the present invention relates to a method of manufacturing a semiconductor device, which includes a laser beam irradiation step. Note that a semiconductor device described here includes an electro-optical device such as a liquid crystal display device or a light emitting device, and an electronic device that includes the electro-optical device as a part.
2. Description of the Related Art
In recent years, an extensive study has been made on a technique in which an amorphous semiconductor film formed on an insulating substrate made of glass or the like is crystallized so that a semiconductor film having crystal structure (hereafter referred to as crystalline semiconductor film) is obtained. As the methods of crystallization such as a thermal annealing method using furnace annealing, a rapid thermal annealing method (RTA method), a laser annealing method and the like were examined. Anyone thereof or combining two or more methods thereof can be carried out for crystallization.
In comparison with an amorphous semiconductor film, a crystalline semiconductor film has extreme high mobility. Since thus, the crystalline semiconductor film is used to form a thin film transistor (referred to as TFT), for example, the TFT can be used in an active matrix type liquid crystal display device etc. in which TFTs for pixel portion, TFTs for pixel portion and TFTs for driver circuit are formed on one glass substrate.
Generally, in order to crystallize an amorphous semiconductor in annealing furnace, a thermal treatment at 600° C. or more for 10 hours or more is required. A quartz is an applicable material of substrate for this crystallization, but the quartz substrate is too expensive in price and is very difficult to be manufactured especially in a large area. In order to improve the productivity efficiency, manufacturing the substrate in a large area is acceptable. There is a reason why the research to form the semiconductor film on the glass substrate that is cheap and easy to manufactured in a large area substrate is advanced in this point. It is expected that a glass substrate in which a length of one side exceeds 1 m will be also used in recent years.
On the other hand, a method of thermal crystallization by using metal elements disclosed in Japanese Patent Application Laid Open No. 7-183540 enable the crystallization temperature, which was a conventional problem, to be realized at a low temperature. The crystalline semiconductor film can be formed by this method in which a small amount of an element such as nickel, palladium and lead is added to an amorphous semiconductor film, then the amorphous semiconductor film is heated for four hours at 550° C. If the heat treatment is conducted at 550° C., there is no change in shape of the substrate etc. since the temperature is lower than the strain point of a glass substrate.
Since the laser annealing method can deliver high energy only to the semiconductor film without substantially increasing the temperature in substrate, the laser annealing technology comes under spotlight by its appliance in a glass substrate with a low strain point as a matter of course, and a plastic substrate, etc.
An example of the laser annealing method is a method of forming pulse laser beam from an excimer laser or the like by an optical system such that it becomes a square spot of several cm or a linear shape of 100 mm or more in length on a surface being illuminated, and relatively shifting an irradiation position of the laser beam with respect to the surface being illuminated to conduct annealing. The “linear shape” described here indicates not a “line” in the strict sense but a rectangular shape (or a prolate elliptical shape) having a high aspect ratio. For example, although, it indicates a shape having an aspect ratio of 2 or more (preferably, 10 to 100), it does not make any difference from that a shape at a surface being irradiated is being contained in the laser light having rectangular shape (rectangular shape beam). Note that the linear shape is used to obtain an energy density required for annealing an object sufficiently to be irradiated. Thus, if sufficient annealing is conducted for the object to be irradiated, it may be a rectangular shape and a tabletop shape.
However, the crystalline semiconductor film manufactured by the laser annealing method is formed from a plurality of crystal grains, and the position and the size of the crystal grains are random. The TFT manufactured on the glass substrate is formed by separating the crystalline semiconductor film with island-like patterning in order to realize element isolation. In this case, the crystalline semiconductor film cannot be formed with the designation of the position and the size of the crystal grains. There exist numerous recombination centers and trapping centers which derive from an amorphous structure, a crystal defect or the like exist in an interface of the crystal grain (crystal grain boundary) compared with the inside of the crystal grain. It is known that when a carrier is trapped in the trapping center, the potential of the crystal grain boundary rises, which becomes a barrier to the carrier, whereby the current transporting property of the carrier is lowered. The crystallinity of the semiconductor film in a channel forming region has a great effect on the characteristic of the TFT. However, it is hardly possible that the channel forming region is formed of a single crystal semiconductor film without influence of the crystal grain boundary.
In recent years, attention has been given to technology according to which crystals are continuously grown in the scanning direction by irradiating a semiconductor film with a CW laser which is scanned in one direction thereby to form single crystalline particles which are extending in that direction. It is considered that this method makes it possible to form the crystals without almost containing crystalline grain boundaries in at least the channel direction of the TFTs. However, this method uses the CW laser of a wavelength region which is sufficiently absorbed by the semiconductor film. The CW laser, however, is capable of producing an output which is as very small as about 10 watts. From the standpoint of production, therefore, this method is inferior to the technology which uses the excimer laser. The CW laser suited for this method has a large output, has a wavelength shorter than that of a visible ray and has a very high stability in the output. Examples may include second harmonics of a YVO4 laser, second harmonics of a YAG laser, second harmonics of a YLF laser, second harmonics of a glass laser, second harmonics of a YalO3 laser and an Ar laser. The lasers of the above harmonics are generally doped with Nd which undergoes excitation to oscillate the laser. The kind of dopant may be suitably selected by a person who executes the method. However, the lasers described above are highly likely to induce interference causing a shading in the irradiation due to interference. Besides, the semiconductor is irregularly annealed depending upon a difference in the angle of incidence of laser beam upon the semiconductor film. This invention was achieved to solve such defects.